The present invention relates to symbol such as barcode reading techniques, and more particularly, to an improved oscillation scan mirror assembly in which the wobbling movement present in prior devices is suppressed and the accuracy of the scan increased. Additionally, the invention, in one embodiment, decreases friction between a shaft and bearing by using a material suited for this purpose.
In an existing oscillation scan mirror assembly 10 as illustrated in FIG. 1a, a scan mirror 1 is secured to a holder 2, which in turn is supported on a stationary shaft 3 through a bearing 4. The bearing 4 has a bearing surface 4a for supporting the holder 2 on the shaft 3, and a recessed portion 4b not in contact with the shaft 3 so as to lessen the friction between the shaft 3 and the bearing 4. An oscillating magnetic driving force is generated by an alternating current passing through a coil 6 wound on a yoke 7, which drives a magnet 5 embedded in the holder 2 to oscillate, thereby causing the holder 2 and the mirror 4 to oscillate around the stationary shaft 3 during a scanning operation.
To allow the holder 2 to oscillate around the shaft 3, there is provided a clearance between the shaft 3 and the bearing surface 4a of the bearing 4. As best seen in FIG. 1b, the existence of the clearance between the bearing surface 4a and the shaft 3 results a tilt of the bearing 4 relative to the stationary shaft 3. During the scanning operation, in addition to oscillating around the shaft 3 in a horizontal plane (i.e., a plane lateral or perpendicular to the axial direction of the shaft 3), the tilting bearing 4 also vibrates in vertical planes (i.e., planes substantially along the axial direction of the shaft 3). This is defined herein as wobbling. Though there may be some lateral magnetic force between the yoke 7 and the magnet 5, it is not large enough to constantly keep the shaft 3 and the bearing 4 in an aligned contact with each other. The undesirable vertical wobble or vibration of the bearing 4 relative to the shaft 3 may result in a tilt of the scanning ray and a double line effect in the laser beam trace, therefore lowering the precision of the scanning operation. The wobbling might also contribute to wearing out of the bearing 4, shaft 3 and other components, and therefore shorten the life of the assembly, although all the adverse affects of such wobbling are not fully known.
A conventional solution to alleviate the above problem of wear is to fill oil at the bearing surface 4a so as to suppress the wobbling movement. However, this could result in leaking and is thus suboptimal. There are no easily available solutions to the problem of wobbling known to the inventors hereof.